[Radiance-general] Calculation of solar declination in sun.c

[Mark Stock]

I hope this contributes to the discussion: the genUtahSky program that
I presented a few workshops ago (top of
http://markjstock.org/radiance/) uses the libnova library for
astronomical calculations. It's a good place to start for all kinds of
astronomical data, though being an external library might not make it
the best choice for this application.

Mark

On 12/17/15, Greg Ward <gregoryjward at gmail.com> wrote:
> Given the tool construction, it would be simple enough to introduce a global
> variable that switches between the de facto solar angle calculation and a
> more precise version.  I'd like to hear from others that this is worthwhile
> before I spend too much time on it, though.  Also, whether it should be
> exposed as an option in all of the calling programs (i.e., IES calculation
> or more accurate on).
>
> Is there a general consensus on which solar calculation is best?  I would
> just end up googling it, unless you or someone else has a strong
> recommendation.
>
> Cheers,
> -Greg
>
>> From: Martin Gut <gut at Transsolar.com>
>> Subject: Re: [Radiance-general] Calculation of solar declination in sun.c
>> Date: December 17, 2015 8:55:38 AM PST
>>
>> Hi Greg,
>>
>> Thank You for the quick answer.  I don’t know about the history of this
>> debate because I’m quite new in this mailing list.
>> I think for the CIE standard stuff, sticking on the standard routines is
>> ok.
>>   But this formula is also used for all the dynamic daylighting stuff
>> like
>> Gendaymtx
>> Gendaylit
>> Daysim
>> etc.
>> also for shading studies: I’m wondering that so much computing effort is
>> investigated to render the shading and on the other side the position of
>> this shading is incorrect (especially in Autumn) because of
>> a very rough calculation of the sun position. The error is not so big, but
>> it could be avoided very easy by using a better formula for solar
>> declination.
>> Maybe there could be two functions, one for the standard calculations and
>> one for “real”  calculations.
>>
>> Martin
>>
>>
>> Von: Greg Ward [mailto:gregoryjward at gmail.com]
>> Gesendet: Donnerstag, 17. Dezember 2015 17:14
>> An: Radiance general discussion
>> Betreff: Re: [Radiance-general] Calculation of solar declination in sun.c
>>
>> Hi Martin,
>>
>> This debate comes up every so often with the solar calculations used in
>> Radiance and related tools.  The main question is, do we stick with the
>> established standard, which provides for easy "apples to apples"
>> comparisons, or do we update our formulas to get a more precise answer?
>> In the case of the original CIE sky models, which this solar calculation
>> is used in primarily, the accuracy is not that great, so getting the sun
>> in exactly the right place is a minor quibble.  If, on the other hand, you
>> need precise sun position for solar shading studies or the like, then it's
>> easy to argue for a better formula.  I don't think computing power was
>> ever the issue with Radiance.  We don't compute the sun position for every
>> ray-traced or anything silly like that.
>>
>> As Rick points out, the use of 368 is part of the IES standard
>> calculation, but I'm not entirely sure what anomaly it is correcting for.
>>
>> Cheers,
>> -Greg
>>
>>
>> From: Richard Mistrick <RGMARC at engr.psu.edu>
>> Subject: Re: [Radiance-general] Calculation of solar declination in sun.c
>> Date: December 17, 2015 7:18:53 AM PST
>>
>> I’m not certain why it is 368, but this equation has been in this format
>> in the IES Lighting Handbook for many years.
>>
>> Rick
>>
>> From: Martin Gut [mailto:gut at Transsolar.com]
>> Sent: Thursday, December 17, 2015 6:05 AM
>>
>> Dear Radiance Experts,
>>
>> I have two questions regarding the calculation of solar declination:
>>
>> 1.       Why has the year in function sdec  in  file sun.c  368 days
>> instead of 365 as in the original formula from Cooper ?
>>
>> return( 0.4093 * sin( (2*PI/368) * (jd - 81) ) )
>>
>> https://github.com/NREL/Radiance/blob/master/src/gen/sun.c
>>
>> 2.       Why does Radiance not use a better formula, which takes into
>> account, that the Earth orbit is not a circle?
>> With today computing power, there is no more reason to use this simple
>> formula
>>
>> Thanks in advance
>>
>> Martin
>>
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